Boiling Flow Instabilities in Parallel Channels

Abstract

The instabilities in a two-parallel-channel forced-convection boiling upward flow system have been investigated using Freon-11 as the test fluid. Two major modes of oscillation, similar to those encountered in single-channel boiling flows, have been identified. The first, the density-wave mode, occurs with relatively high exit vapour fractions for one or both of the channels, and also when the working fluid is completely evaporated and superheated. The second, the pressure-drop mode, occurs when the pressure drop across the test section decreases with increasing flow rate and the exit vapour fraction for one of the channels is low. Whenever both of the criteria are met, the two modes of oscillation are found to be superimposed on each other. The system could be stabilized by introducing pressure drop(s) at heater inlet(s) of either channel or both channels. An analytical study has been made of the pressure-drop oscillations by linearizing the governing equations and using Routh's criterion. The conditions for occurrence of these oscillations are predicted by the theoretical analysis.